1. A group of software whose source code is available to the general public for the purpose of improvement, modification, etc.
2. Any software released under the GPL, LGPL, or other open source licenses.
3. The stuff that makes Bill Gates' life sad.

Linux, Apache, MySQL, and plenty else is open source - and it's all good stuff.

Giving the public the ability to modify, improve, and fix the program itself. Allowing the creator of the program to not have to find a way to fix the program himself and allowing more patches and improvements to the software.

Also allowing the public to be able to port the program onto other operating systems, so that dual-booters who can program will make a windows version of an awesome GNU/Linux/BSD/Solaris program.